In a widely used form of electric arc furnace, several, usually three, consumable graphite electrodes extend substantially vertically downwardly from electrode clamps, through apertures in the roof of the furnace (if a roof is provided) and into the furnace charge. In order to strike and maintain a proper arc during operation of the furnace, it is necessary to allow for significant vertical movement of the individual electrodes, whilst in the case of steel making furnaces, it must be possible to withdraw the electrodes sufficiently upward relative to the furnace roof to allow the latter to be swung clear of the remainder of the furnace structure. It must also be possible to feed the electrode downwardly relative to the clamp to compensate for erosion of the electrode tip during use.
Various problems are associated with the facts that a substantial distance separates the effective lower tip of each electrode and its supporting clamp, and that substantial vertical relative movement is necessary between the electrodes and the furnace roof. Gases may escape from the furnace between the electrode and the furnace roof, or if the furnace is operated at subatmospheric pressure, cold air may be drawn in through the same gap. The outer surface of the electrode is subject to erosion through oxidation caused by the hot gases within the furnace, and this problem is aggravated by heating of the electrode caused both by heat from the furnace interior and by electrical heating caused by the electrical resistance of the electrode to the current passing therethrough.
Two main approaches have been adopted in response to these problems. Firstly, various forms of telescoping seal have been proposed for use between individual electrodes and the furnace roof. For example, in French patent No. 1,418,153 (IRSID), a sleeve extending downwardly from the electrode clamp moves vertically in a water filled annular chamber extending upwardly from the furnace roof. In U.S. Pat. No. 4,306,726 (Lefebvre) a series of concentric sleeves with sliding seals provides a telescoping gas seal between the electrodes and the furnace roof. Both of these arrangements have the disadvantage that it is not practicable to accommodate the large range of vertical electrode movement required in a steel making furnace, whilst a major part of the weight of the seal structure must be supported by the furnace roof in an area where it is normally sought to avoid unnecessary loadings.
The second approach has been to seek reduction of heating of the electrode. Numerous proposals have been made for fluid (usually water) cooled electrode structures, which reduce the length of graphite electrodes required. Examples may be found in U.S. Pat. Nos. 4,121,042 and 4,168,392 (Prenn), 4,287,381 (Montgomery), French Patent No. 1,418,153 referred to above and French Patent Publication Nos. 2,176,546 and 2,222,821 (IRSID), German Patent Publication No. 2430817 (Sigri), British patent specification No. 1,223,162 (Ostberg) and Belgian Patent No. 867,876 (Korf-Stahl AG). All of these proposals have in common that the cooling arrangement is more or less complex and expensive, and that it is not possible continuously to add additional electrode sections to the top of the electrode to replace erosion from the bottom of the electrode. Rather, the entire electrode assembly must sooner or later be removed in order to replace the tip portion if this is consumable.
In British Patent Specification No. 664,298, an arrangement is proposed in which the electrode is supported within a water cooled sleeve, the lower end of which is in electrical contact with the electrode, and the upper end of which is held by the electrode clamp instead of the electrode itself. In this arrangement, the portion of the electrode within the sleeve does not have to transmit electric current, and the outer surface of this same portion is shielded from the electrode gases. In practice, it is extremely difficult to maintain adequate electrical contact between the bottom of the sleeve and the electrode. The arrangement described relies upon external screw threading of the electrode, both for transmitting current and for advancing the electrode through the sleeve, which means that electrodes of conventional design cannot be employed. Moreover, in order to achieve the object of the invention, the sleeve must extend close to the tip of the electrode. This is not practicable in many applications because of the risk that the sleeve will be damaged or destroyed by arcing from the furnace charge. An alternative approach is found described in U.K. Patent Specification No. 2000947A (Korf-Stahl AG) in which a water cooled jacket is provided around the electrode so as to shroud and support the entire electrode apart from the tip. In order to protect the jacket from arcing, it is insulated from the electrode, and provided with a refractory cladding. Additionally, an arc control magnet must be provided so as further to reduce the risk of arcing. The sleeve is supported from an electrode support arm which supports the electrode through a drive mechanism which also must maintain insulation between the electrode and the arm, a separate electrical connection being made to the electrode. It is suggested that the sleeve might be supported from a conductive electrode clamp, but in this case some arrangement would be necessary to isolate the sleeve electrically from the clamp. In order to maintain insulation between the sleeve and the electrode it is necessary to provide guides for the electrode within the lower part of the sleeve in a vulnerable position near the tip of the electrode.